Unless the collection is empty, the index must exist prior to the
shardCollection command. If the collection is empty,
MongoDB creates the index prior to sharding the collection if the
index that can support the shard key does not already exist.

When true, the unique option ensures that the underlying index
enforces a unique constraint. Hashed shard keys do not support unique
constraints. Defaults to false.

numInitialChunks

integer

Specifies the number of chunks to create
initially when sharding an empty collection with a hashed
shard key. MongoDB will then create and
balance chunks across the cluster. The numInitialChunks must be
less than 8192 per shard.

Changed in version 3.4: If the collection is not empty or the shard key is not a hashed
key, the operation returns an error.

collation

document

Optional. If the collection specified to shardCollection
has a default collation,
you must include a collation document with
{locale:"simple"}, or
the shardCollection command fails. At least one of the indexes
whose fields support the shard key pattern must have the simple
collation.

Do not run more than one shardCollection command on
the same collection at the same time.

MongoDB provides no method to deactivate sharding for a collection
after calling shardCollection. Additionally, after
shardCollection, you cannot change shard keys or modify
the value of any field used in your shard key index.

If the collection has a default collation,
the shardCollection command must include a collation parameter with the
value {locale:"simple"}. For non-empty collections with a
default collation, you must have at least one index with the simple
collation whose fields support the shard key pattern.

You do not need to specify the collation option for collections
without a collation. If you do specify the collation option for
a collection with no collation, it will have no effect.